William Ames

Project: I worked measuring contact resistances and electron mobility in graphene devices, as part of a broader project to develop graphene-device quantum Hall resistance standards. The quantum Hall effect is a quantum mechanical analog to the classical Hall effect, where a conducting bar with a current running through it and a magnetic field applied perpendicular to the face of the bar develops a voltage difference across the bar, orthogonal to both the magnetic field and the current, due to the magnetic force on the charge carriers. A ratio of a voltage and a current is a resistance, and thus we can define a Hall resistance as the ratio of the Hall voltage to the current. In a perfectly two-dimensional bar, under high magnetic fields and low temperatures, this resistance becomes quantized, a consequence of the quantum Hall effect. These quantized resistances are extremely stable and reproducible, with their value determined by fundamental constants of nature, which is why they are the basis of the definition of the Ohm. However, the apparatus necessary to perform calibrations with a quantum Hall standard is very difficult and time consuming to use, due to the high magnetic fields and extremely low temperatures necessary, making standards propagation something of a difficult task.

Graphene, a recently discovered allotrope of Carbon, could potentially be the solution to this problem. Graphene is a single layer of hexagonal rings of Carbon atoms, all tightly bound together. Multiple sheets of graphene stacked on top of each other and loosely bound together form the familiar graphite. Graphene has many exciting physical and electrical properties, and is the subject of intense research. Most importantly from the standpoint of resistance metrology, it is perfect two-dimensional plane, making it an ideal material of quantum Hall devices. The Gallium-Arsinide devices which are the current state of the art in resistance metrology are actually stacks of several materials on top of each other, which behave like a two dimensional object at low temperatures and high magnetic fields. However, because graphene actually is two-dimensional, it is potentially a much better material for quantum Hall devices. A graphene quantum Hall device could be used in a standard liquid Helium cryostat, with no need for milli-Kelvin temperatures, and under relatively normal laboratory magnetic fields, both of which make it much easier and cheaper to use. Since ease of propagation is an important consideration in standards work, this makes graphene quantum Hall devices a potentially dramatic improvement over the state of the art.

The downside to graphene devices is they have very large contact resistances. These high contact resistances lead to noise in the resistance measurements, even when making four-point measurements. In order to create a viable quantum Hall standard, a way to produce graphene devices with low contact resistances must be developed. My work over the summer was focused on bringing into operation a cryostat which will be used to make measurements of contact resistance and electron mobility, which gives important information about the electrical properties of the sample, in graphene devices as part of the overall effort to reduce these contact resistances.

Over the course of the summer, I took an old cryostat, which had been left out open to the elements for many years, and reworked its wiring to be compatible with our needs, and designed and built internal hardware for the mounting of samples and data collection. I also wired and installed two Silicon diode thermometers. I cleaned the cryostat sufficiently that it pumped down to pressures of approximately 10-6 Torr, and made sure the cryostat could hold both liquid Nitrogen and liquid Helium without any problems. The cryostat is now fully functional, and can measure resistances and electron mobilities. All that remains to be done is take some lead resistance measurements, for which the apparatus and procedure have already been constructed and designed, and the cryostat can go into full operation.

About Me: I graduated with a BS in Physics and a minor in Classical Civilization from the College of William and Mary May 2009, and will be attending the University of Colorado at Boulder starting in August 2009 working towards a doctorate in Physics. My primary research interests are in atomic physics, particularly quantum optics, though I hardly restrict myself to that field, as my experiences this summer indicate. My first experience working with a scientist was in the SURF program in 2006, when I worked with Dr. Volkovitsky of the Ionizing Radiation group on radon diffusion, and that experience has certainly encouraged me and convinced me that physics is what I should do with my life. My non-science interests are primarily Classical history and literature and playing the harpsichord.